As shown in FIG. 1, a direct current side of a five-level inverter 10 is connected to a photovoltaic array 20 and configured to receive a direct current PV input voltage. An output voltage from a grid side of the five-level inverter 10 is filtered by a filter 30, step up by a transformer 40 and then transmitted to a power grid. The PV input voltage is applied across a positive electrode of a capacitor C1 and a negative electrode of a capacitor C2, voltage applied across C1 is V1Pos and voltage applied across C2 is V1Neg. And the PV input voltage is step up by two Boost circuits, then is applied across a positive electrode of a bus capacitor C3 and a negative electrode of a bus capacitor C4, and voltage applied across C3 is V2Pos and voltage applied across C4 is V2Neg. In different combinations of switching states of switching devices, the five-level inverter 10 alternately outputs levels +V1Pos (i.e., +1), −V1Neg (i.e., −1), +V2Pos (i.e., +2), −V2Neg (i.e., −2) and a zero level corresponding to the middle point of the DC bus.
In a case that the PV input voltage is higher than a bridge line-line voltage command value of the five-level inverter 10 required when the five-level inverter 10 is connected to a power grid, the operation mode of the five-level inverter 10 is switched automatically from a five-level operation mode to a three-level operation mode. However, since in this case the bus voltage is high and the output voltage level of the grid side inverter is reduced, power loss of switching devices S1Pos, S1Neg and S0 increases significantly, and a junction temperature of the switching devices is higher, over-temperature protection for the switching devices may be triggered and service lives of the switching devices are reduced greatly.